Commitments and Contingencies - Additional Information (Detail)			1 Months Ended	6 Months Ended
	Apr. 21, 2017 USD ($) ft²	Sep. 30, 2016 USD ($) ft²	Sep. 30, 2013 USD ($)	Jun. 30, 2017 USD ($) $ / ft²	Dec. 31, 2014 USD ($)	Jun. 30, 2014 USD ($) ft²	Dec. 31, 2013 ft²
Bluebird [Member]
Long-term Purchase Commitment [Line Items]
Lease agreement effective date				May 01, 2017
150 Second Street [Member]
Long-term Purchase Commitment [Line Items]
Area of office and laboratory space under lease agreement | ft²							61,591
Lease expected term				8 years
Lease rent waiver period				10 months 15 days
Value of rent obligations			$ 3,300,000
Security deposit of lease agreement			1,725,000		$ 864,000
Tenant improvements			$ 9,239,000
150 Second Street [Member] | Second Amendment [Member]
Long-term Purchase Commitment [Line Items]
Tenant improvements						$ 1,020,500
Additional space for lease | ft²						8,164
Lease commencement date				2014-11
150 Second Street [Member] | Third Amendment [Member]
Long-term Purchase Commitment [Line Items]
Area of office and laboratory space under lease agreement | ft²		123,210
Security deposit of lease agreement				$ 1,771,009
Lease rent abatement period				2 months
Lease expiration date				Apr. 30, 2024
Extendable/extended lease term				5 years
150 Second Street [Member] | Third Amendment [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Tenant improvements to leased space		$ 2,500,000
Annual base rent payable per square foot | $ / ft²				83.42
150 Second Street [Member] | Third Amendment [Member] | Minimum [Member]
Long-term Purchase Commitment [Line Items]
Annual base rent payable per square foot | $ / ft²				70.51
150 Second Street [Member] | Third Amendment [Member] | Bluebird [Member]
Long-term Purchase Commitment [Line Items]
Area of office and laboratory space surrendered under lease agreement | ft²		53,455
150 Second Street [Member] | Bluebird Premises [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Annual base rent payable per square foot | $ / ft²				72.84
150 Second Street [Member] | Bluebird Premises [Member] | Minimum [Member]
Long-term Purchase Commitment [Line Items]
Annual base rent payable per square foot | $ / ft²				62.83
One Kendall Square [Member]
Long-term Purchase Commitment [Line Items]
Lease expected term				7 years 6 months
Rent abatement				$ 1,841,000
525 University Avenue [Member] | Second Amendment [Member] | PAOC [Member]
Long-term Purchase Commitment [Line Items]
Area of office and laboratory space under lease agreement | ft²	1,975
Lease expiration date				Apr. 30, 2022
Extendable/extended lease term				60 months
Lease rent per month	$ 17,775
Increase in annual lease rent, percentage	3.00%
Lease rent abatement period				2017-05